An important requirement to the electric drive of the above type consists in the maintenance of synchronism of displacement of suporting members moving along equidistant trajectories. Numerous attempts to meet this requirement are known.
Thus, known in the art are electric drives for synchronous rotation of mechanically independent shafts (M. G. Chilikin et al. "Teoriya avtomatizirovannogo elektroprivoda"--"Theory of automated electric drive", Moscow, "Energiya", 1979, pp. 538-557). In the above electric drives each shaft is mechanically coupled with a separate synchronous or asynchronous electric motor. Windings of rotors of these motors are electrically connected therebetween.
In the case of a lag of one of the shafts, which would result in the angular deflection of the supporting members, a corresponding motor produces an additional torque to eliminate misalignment, and consequently to a certain degree prevent the angular deflection of the supporting members.
Among the above specified electric drives, the most typical is an electric drive including two d.c. electric motors located equidistantly and electrically connected therebetween (V. P. Andreev, Yu. A. Sabinin. "Osnovy elektroprivoda"-"Fundamentals of electric drives", Moscow-Leningrad, Gosenergoizdat, 1963, pp. 674-675). Each of these motors includes a stationary former of magnetic field, a movable portion disposed within this magnetic field, an excitation winding, a sectionalized armature winding, and a collector with brushes, commutating the sections of the armature winding.
To synchronize the rotation, d.c. electric motors are provided with additional contact rings whereto are connected three symmetric points of the armature winding, shifted relatively one another by 120 electrical degrees. The rings of these motors are electrically connected therebetween. From the viewpoint of the contact rings, said electrical machines operate like placed-in-parallel synchronous generators. In the case of occurrence of a misalignment, a circulating current starts flowing over the armature windings, due to which fact there occur synchronizing torques retaining the machines in the synchronous rotation mode. However, these torques decrease with decreasing speed of rotation of the armatures. The speeds of rotation of the armatures being low, when the electromotive force induced in the armature windings is practically equal to zero, the synchronizing torques also practically vanish, which fact makes inexpedient the use of such an electric drive for shifting lengthy gantries of e.g. thermal cutting machines, where in the displacements at very low speeds, of the order of tens of millimeters per minute, the main processing operation modes are practiced.